Any old Magic Band will work, unless you have deactivated it in
MDE (something you would have to have done on purpose).
Magic Bands have an internal battery that lasts “a couple of years” that supports long-range functions such as ride photos and Disney being able to know where you are. But a Magic Band will work forever on short-range functions such as tapping admission into the parks, tapping on your room door (aside from an occasional resort room door issue which has to do with the resort’s computer system, as what has happened to some of us above).
It’s important to remember that nothing is ever “on” a Magic Band - it just serves as an access point to your MDE profile. So your current MB, one from last year, or one from three years ago, etc, all do the same thing - access your profile.
We usually bring a few just in case we lose one, some in our party want to match outfits, etc.
Hope that helps!